A concatenated code is a doubly-encoded type of code having an inner code and an outer code. The inner code encodes the outer code.
The inner code corrects most of the errors introduced by the communications channel and is typically a convolution code. The outer code corrects the majority of decoding errors (which typically are bursty) that occur during the first decoding. The Reed-Solomon (R-S) code is commonly used as the outer code.
FIG. 1 is a block diagram of a typical concatenated code system 100 according to the Background Art. The system 100 includes: an outer encoder 102; an optional interleaver 104; an inner encoder 106; a modulator 108; a communications channel 110; a demodulator 112; an inner decoder 114; an optional de-interleaver 116 (present if the interleaver 104 is present); and an outer decoder 118.
FIG. 2 is a block diagram of a concatenated code iterative decoder 200 according to the Background Art that is compliant with the Advanced Television Standards Committee (ATSC) 8-VSB (vestigial sidebands) standard. The decoder 200 includes: a first inner, trellis-coded modulation (TCM) decoder 202; an optional de-interleaver 204 (that itself has a symbol de-interleaver 206 and a convolutional de-interleaver 208); a first outer, R-S decoder 210; an optional interleaver 212 (that itself has a convolutional interleaver 214 and a symbol interleaver 216); an inner TCM encoder 218; a second inner TCM decoder 220; a second optional de-interleaver 222 (present if interleaver 212 is present) (the de-interleaver 222 including a symbol de-interleaver 224 and a convolutional de-interleaver 226); and a second outer R-S decoder 228.
The decoder 200 is designed to work with a channel exhibiting additive white Gaussian noise (AWGN). The ATSC standard for 8-VSB requires a decoder to successfully decode a signal that is received having a maximum segment error rate (SER) of 1.93×10−4 at a minimum energy per symbol (ES/NO), or signal to noise ratio (SNR), of 14.9 dB.
The decoder 200 will decode a signal having an SNR of 14.9 or greater. For an SNR below 14.9 dB, the decoder 200 will fail to decode the received signal, yet such failure does not prevent the decoder 200 from being considered ATSC-compliant.